THE PAST EVOLUTION OF MAN 73 



difference. It is known that many cases of human 

 idiocy are associated with a small size of the head 

 (microcephaly). It is further found that in such 

 cases the sutures or joints between the various 

 cranial bones close unusually early; and, since the 

 cranial bones grow in area by addition of new bone 

 tissue along their edges, this premature fusion of 

 the edges arrests further growth. The inference 

 naturally suggests itself that it is this premature 

 closure of the cranial sutures which must be held 

 to account for the small size of the brain ; which 

 is, as it were, thus clamped down in an inextensible 

 box. Hence it has been suggested that the im- 

 mediate cause of the greater size of the human, as 

 compared with the simian, brain is to be sought in 

 the later closure of the cranial sutures in the case 

 of man. 



When, however, we come to examine this widely- 

 accepted hypothesis more closely, we find it to lack 

 the truly scientific character. 1 Why should the 

 cranial sutures close later in man ? The hypothesis 

 which professes to explain the facts offers no ex- 

 planation of the cardinal fact. Further, it is to be 

 remembered that the brain — as we know beyond 

 dispute — controls the growth J and functions and 

 activities of all parts of the body, the skull included, 

 in virtue of what we call its trophic power. Hence 

 we must amend our hypothesis and declare that 

 the brain is the author of that late or early closure 

 of the cranial sutures which reacts upon its own 



1 Operations for relief of microcephalic idiocy by excising strips 

 of cranial bone and so "allowing the brain to grow" have been 

 uniformly unsuccessful. 



